In Hodgkin's Disease (HD), the most common lymphoma in the Western world, the malignant cells are the so-called Hodgkin and Reed-Sternberg (HRS) cells, comprising only a few percent of the lymphoma mass. HRS cells are often infected by Epstein-Barr-Virus (EBV) and express the EBV proteins LMP1 and LMP2A, partially mimicking constitutively an active CD40 co-receptor and B cell antigen receptor (BCR), respectively. Previous work, which had identified somatically mutated B cells, often bearing mutations crippling BCR expression, as HRS progenitors, led to a scenario of HD pathogenesis, in which HRS cells derive from pre-apoptotic GC B cells rescued by LMP2A and LMP1 expression. Despite their B cell origin, HRS cells have largely lost the B cell-specific gene expression program and acquired expression of genes typical for the T and/or myeloid hematopoietic lineages. We hypothesize that this lineage infidelity is dictated by the interference of certain transcription factors (TFs) known to be expressed in HRS cells, namely Id2, ABF1 and Notch1, with the B cell- specific gene expression program. This together with constitutive expression of proteins promoting cell survival and proliferation, like the TF c-Jun and TFs of the NF-kB family, may ultimately reprogram GC B cells into HRS cells. In this general scenario, EBV infection is a major initial transforming event in HD, and is indeed known to result in up-regulation of the TFs NF-kB, AP1, Id2 and ABF1 in B cells. Following this general hypothesis, we have developed a strategy to target the expression of candidate genes into GC B cells, using Cre-mediated conditional gene targeting. Using and further improving these tools, we plan to analyze whether induced expression of the various EBV-derived proteins and TFs individually and in combination in GC B cells will result in trans-differentiation and transformation of those cells as it is seen in HRS cells in HD. Our ultimate goal is to generate a mouse model of HD and to better understand the role of cellular reprogramming in lymphomagenesis. A second EBV-associated disease addressed in the present proposal is Post-Transplant Lymphoproliferative Disorder (PTLD), common in post-transplantation patients and due to a significant extent to the outgrowth of EBV-infected B cells because of immune suppression. We have found that induced expression of the EBV protein LMP1, known to be a major player in EBV-mediated B cell transformation, in developing B cells in the mouse leads to the rejection of these cells by the immune system. Depletion of T cells in the animals results in the rapid outgrowth of LMP1 expressing B cell blasts and death of the mice within a few weeks. We plan to develop this system into a first mouse model of PTLD, by targeting LMP1 expression into mature B cells, and to fully characterize disease progression and reversibility in the mutant animals, with a view of ultimately using this model for the development of new therapeutic strategies.